The reason is simple: The question specifies a version of Acrobat Reader. It even did so back in April 2015 when it was originally posted.
An answer for a much later version that works differently from the version being asked about isn't an answer to the question that is being asked, and thus by definition cannot be the most useful answer to the question that is being asked.
If you are happy with the question being about "any version of Acrobat Reader with a sidebar superficially similar to the one in 2015.007.20033", then the question should be edited to explicitly state that, or at the very least not explicitly state a version number other than for the purposes of being an example.
If you want to keep the question specific to the version that you originally asked about, the person who posted the answer should be encouraged to post it as a self-answer to an updated question for the version that the answer is about, preserving the integrity of the original question.